1. Field of the Invention
The present invention relates to a method and apparatus for producing a blown film, and more particularly to a method and apparatus in which an improved bubble stabilizer is employed.
2. Description of the Prior Art
In a blown-film extrusion for producing a tubular resin film, it is important to prevent molecular orientation and thickness deviation from occurring in the film in order to produce a uniform film, which molecular orientation and thickness deviation result from material deformations occurring in flow or extension of the resin material which is extruded from a die in a molten state and solidified. As shown in FIG. 11, one such preventive measure is disclosed in a blown-film extrusion described in Japanese Patent Publication No. 55-2180 in which a bubble is extruded from a die while being brought into contact with a small-diameter bubble stabilizer which projects from a surface of the die.
However, in such conventional blown-film extrusion, since a tubular bubble stabilizer employing a bubble stabilizer lacks cushioning properties in its portion contacting a tubular molten material, such stabilizer can not adapt itself to the changes of the bubble's shape corresponding to the changes in room temperature, cooling effect, resin temperature, amount of the resin extruded, thickness of the bubble and extrusion speed of the film, so that the extruded film is often broken by occurrence of knocking. In addition, in the conventional blown-film extrusion, the molten resin extruded from the die is cooled by cool air supplied from an air ring while being extruded into a tubular molten resin element. However, such cooling action depending only on the cooling through the air ring restricts the amount of the resin extruded and therefore is not adequate in increasing the amount of production the film per hour.
Further, in the conventional blown-film extrusion, since the bubble stabilizer is fixed in its size and shape, it is necessary to provide various tubular bubble stabilizers having various sizes and various shapes corresponding to various sizes and thickness of the blown films to be produced. Consequently, various types of dies are required which results in a cumbersome maintenance which is another problem inherent in the conventional blown-film extrusion.